The answer to the question, what is the most efficient heating system, is that there isn’t one. That is, not ONE. There are, of course, efficient heating systems but no ONE that will suit every home – low energy or not. It also depends on how we define “efficient”. It is like asking what is the best car. Put 10 car enthusiasts in a room and they will come out with 25 or 30 opinions. And they could all be right. A definition of “best” will vary with what the car has to do, what the owner likes in a car and the budget available. In a younger life, I needed to transport 3 children and a couple of dogs. A Lamborghini Countach was a dream, but never an option. A VW Passat estate filled the bill but was it ever the “best” car?

Efficiency in a heating system is very similar; it is a factor of many things – what the heating system has to do (or how much heat it needs to produce), what the owner likes in a heating system (convenience or running cost) and the budget available. Efficiency is often measured purely in terms of fuel consumption or running cost (those two may seem the same but they can be very different). But we could also factor in return on investment.

Renewable energy systems are significantly more expensive to buy than fossil fuel systems. But we get money back from the government in the form of Renewable Heat Incentive. It is inarguable that a ground source heat pump delivers more heat output for each kWh of electricity input than an air source heat pump does, but capital cost is much higher. If the property does not need that extra heat, is the ground source the more efficient heating system?

Similarly, wood pellets deliver more heat per kilogram than logs do. As a consequence wood pellet boilers are cheaper to run on a cost per kWh basis than log boilers. But if you happen to have a few acres of woodland and are happy to coppice it and properly dry the logs then does the cost per kWh argument still hold true? What is the time invested in harvesting fuel worth?

Deciding on the best, the most efficient heating system has to start with what it has to do – that is, how much heat the property needs. Which needs a proper calculation of the peak demand – that is, the amount of heat in kW that the property will need in the most adverse weather conditions. Typically a heat demand calculation is based on a winter temperature of -2°C. Obviously, there are occasions when the temperature falls below that figure but they tend to be rare and prevail for only a short time.

It is generally considered that designing a heating system to deal with those cold-snaps would be over-engineering. We rely on the inherent slack in any system to deal with those times. That tells us the capacity, or size, of the heat source we need (say, 10kW heat pump, or 25kW wood pellet boiler, for instance). We also need to calculate the annual heat demand in kWh as that tells us the amount of fuel we will need and the probable running cost.

With those two figures in hand, we can begin to make an informed decision as to what the realistic options are and which will suit best. It also leads to the thought that maybe those figures can be reduced. Maybe it would be possible to improve insulation and/or air tightness and thereby reduce the capacity of the heat source and thereby both the capital and running costs.

Taken to the current extreme, Passivhaus, the property, notionally at least, needs no heating system at all. Now that would be really efficient, but it is not actually true. Even Passivhaus needs some form of heating to deal with those cold-snaps. Typically that would be a very small air source heat pump. But it could be solar energy. It is entirely possible to design a house to be heated entirely with solar heating.

It is generally in the form of a solar thermal array on the roof and the effective use of passive solar energy – that is large areas of south-facing glazing to let the sunlight and heat in, and thermal mass in the form of stone or concrete to absorb that heat and release it back into the room as the temperature falls.

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Which brings us to another issue. Heat is needed in the home in two forms; space heating and domestic hot water. These two are quite different in that they require different temperatures and different amounts of heat. To clarify, domestic hot water needs to reach at least 64°C to kill bacteria (Legionella mostly) where an underfloor heating system will need water at around 40°C.

A 4-person household will need about 3,000kWh of heat for domestic hot water but the average 3-bedroom semi might need 10,000 to 15,000kWh of space heating each year. A gas or oil-fired boiler will deals with those differences quite easily, often by over-engineering. A 3-bedroom semi, build to current building regulations, is likely to need a boiler with less than 8kW capacity but you will struggle to find one that small.

It costs the manufacturer no more to make a 10kW boiler than it does to make a 6kW boiler. And the 10kW boiler has more potential customers. If all you need is 6kW but have a 10kW boiler it will always be running inefficiently as it will never get up to its optimum running temperature. It is like using a Transit van to pop down to Tesco’s to get a loaf of bread and a pint of milk.

The reality is that with gas and oil-fired boilers it does not make a huge amount of difference; capital cost difference is not significant and the extra running cost will not be enough to bankrupt anyone. (Having said that, old gas or oil boilers – more than 10 years old – can be really very inefficient and a new one could produce significant savings).

With renewable energy systems, it becomes really important to get the right size; capital cost is always higher and increases with capacity. Heat pumps especially need to be the right size, ideally slightly under-sized, so that they reach their optimum running temperature as quickly as possible and stay at that temperature for as long as possible.

What’s best for a high energy home

The trite answer would be to sort out the insulation and air tightness and make it a low energy home. But it has to be accepted that that is not always practical or possible. Big old country houses will always need a lot of heat and it is likely that biomass – in the form of wood pellets of logs – would be the best technology.

Capital cost is high but they are good at putting out high volumes of heat at high temperatures. It is also a solid, robust technology that will maintain a high level of efficiency for a long time – 20 years plus.

Heat pumps might be an option, but here we get into another controversy. Heat pumps run on electricity and the vast majority of houses have a single-phase (220 volts) electricity supply. If the heat pump output capacity is more than 12kW it is likely that a 3-phase (440 volts) supply will be needed.

Upgrading from single-phase to 3-phase tends to be prohibitively expensive. The sort of house we are talking about is likely to need more than 12kW. Likely more than 20kW. Some experts say that linking two single-phase heat pumps works well. Personal experience indicates that is not always the case – but maybe that is just me.

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The point is that our imaginary house that needs 20kW only needs that in winter – probably December and January – when the outside air temperature plunges. For the rest of the year, it will be happy with 8, 10 or 12kW. A 12kW modulating heat pump will deal with that quite comfortably (modulation is a technology whereby heat output is optimised to match demand so that the heat pump operates just as efficiently at 6kW as it does at 12kW).

But we need to get to 20kW which means that the second heat pump needs to kick in. Then does that heat pump modulate down to whatever the extra demand is or do both heat pumps modulate to match each other? It can be done but it needs a clever control system to do it and a particularly clever installer. My experience has been that these two things do not always come in a happy union.

What’s best for a low energy home

Again it depends on how low. And it can be more difficult to deal with medium-low heat demands than high heat demands, in an efficient way. Very low energy homes, Passivhaus level, need no heating for 10 or 11 months of the year, except domestic hot water.

That is often best dealt with using a solar thermal array, but if that is not possible then the most “efficient” option would be an immersion heater running on overnight electricity tariff (using a renewable energy electricity supplier, of course). It is a very low capital cost, relatively low running cost and sustainable.

A solar thermal array will be a higher capital cost but if it is backed up with overnight electricity it would deal with space heating as well.

A common option for low energy homes is the air source heat pump and in that situation, they make good sense. Relatively low capital and running costs, capable of dealing with space heating and domestic hot water. But they are not the panacea that they seem to be touted as. Move to a medium low-energy house – a house with 8kW peak demand with 4 people living in it – and it will struggle to deliver both space heating and domestic hot water, and maintain a good level of efficiency.

In that case, running the heat pump in conjunction with a gas boiler will work well. Capital cost rises significantly but both machines are running at maximum efficiency – the heat pump in summer and the gas boiler in winter.

Conclusion

Unless you know what you want it is very difficult to get it (witness Brexit negotiations). If we are thinking of a gas or oil boiler the local plumber will put in a boiler that is too big for the house. They always do, so they don’t get any complaints about it being too small. With renewable energy systems, the key is to figure out how much heat – for both space heating and hot water – that the house needs. Then look at the technologies able to deliver it and match that to the budget available.

All of this, of course, after you have done everything humanly possible to reduce the energy demand. That is the best, most efficient, of all options.

We would like to thank Tim for sharing his expertise on our blog. If you would like to speak to him about your low energy home project, he will be available for one on one consultation in the Advice Centre at the Homebuilding & Renovating Shows in 2019.

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